Math

So what's best way to farm incursions?Reload mission until incursion occurs
As reloading mission was too easy Playdom introduce requirements for incursions reveals. It's basically a cumulative score for completed events in mission they are as folow:

Conclusion

With release of chapter 2 seems it was made easier. Each mission in Chapter 2 shows incursion after one medium or hard fight. So the winner here is Chapter 2 Premium Mission. For 30 Energy you can farm two incursions. That is if you already have Scarlet Witch.
If not that lucky then just grab any other mission from Chapter 2 or follow below hints for Chapter 1.